Just before Christmas, on December 12, astronomers announced a new eruption of the most extreme Nova system known to date. The star with the catalogue name "M31N 2008-12a" resides in the Andromeda galaxy - the 2.5 million lightyears distant neighbour of our own Galaxy. Since its discovery in December 2008, this Rapid-Fire Nova has surprised researchers with eruptions more frequent than ever seen before; swiftly becoming one of the most popular observing targets. Astronomers at ICE are playing a leading role in an international team that is at the forefront of this exciting new research.
Recurrent Novae - Regular Fireworks
Nova outbursts are among the most powerful eruptions in the universe. They occur in binary star system consisting of a star like our sun (or sometimes an aging giant star) and a compact White Dwarf star orbiting one another at a relatively close distance. The dense and heavy dwarf star gradually steals material from its companion until it has accumulated enough matter to ignite a spectacular explosion. Within hours, a shell of previously collected material is ejected again at high speeds and temperatures, creating a huge, temporary “new star” - the eponymous Nova that lets the star shine hundreds of thousands times brighter than before. As the shell expands further it cools and fades away, making the star disappear back into obscurity. Unlike the more powerful Supernovae, a Nova does not explode its host White Dwarf but only throws off the accumulated shell. Soon after, the White Dwarf begins to collect new material towards the next eruption. The time between eruptions can be years or millenia. Novae that have shown more than a single eruption are called “Recurrent Novae”.
The most reliable Eruptions in the Universe
One recently discovered Recurrent Nova is breaking all the records: “Our new Rapid-Fire Nova erupts at a much faster rate than any other Nova”, says Martin Henze, a postdoctoral researcher at the Institute of Space Sciences (IEEC-CISC), who is one of the leaders of the global collaboration which studies this fascinating object. In astronomical terms, the eruptions are also exceptionally predictable. “We observe a new event every 350 days”, Henze explains, “and rarely it happens more than a few weeks away from the predicted date.” The opportunity to study a large number of eruptions within only a few decades opens up completely new avenues for Nova research. Professor Margarita Hernanz, working with Henze at the Institute of Space Sciences, emphasises the large impact of this discovery: “The evolution of the binary star system through many, many Nova cycles is something that we could always study only in computer simulations. Now, for the first time, we can actually observe it in real time.”
Towards a spectacular culmination?
Its clock-work like reliability over the last nine years has firmly established the Rapid-Fire Nova as a promising research target for decades to come. But what lies ahead for the binary system? Ultimately, the White Dwarf could gather enough material from its companion star to be pushed beyond a critical mass and explode as a bright Supernova - tens of thousands of times more powerful than the already luminous Nova eruption. Supernovae are critically important for many frontier research fields in astronomy today, from Cosmology to Dark Energy. How a star reaches its critical mass to become a Supernova, however, is still one of the big enigmas. Inspired by the new discovery of the Rapid-Fire Nova, recent theoretical models predict that this system might reach its critical mass within the next million years - a short time in astronomical terms. This makes it the best pre-explosion candidate known today.
Over the coming years, further observations are needed to tie down the model parameters and understand the physics of this exceptional Rapid-Fire Nova. Together with their international team of researchers, Henze and Hernanz are building an astronomical legacy. They are excited by the prospects and by a self-imposed challenge that is unusual for a Nova researcher: “Normally, people wait decades for a Recurrent Nova to erupt again. Now, we want to make sure that we publish the new results before the next eruption happens.”, Henze concludes. This year, the team managed to achieve this goal with remarkable precision: An extensive study of the 2015 eruption was published by The Astrophysical Journal on December 13, the day after the 2016 eruption provided an early Christmas present.
M. J. Darnley, M. Henze, M. F. Bode, I. Hachisu, M. Hernanz, K. Hornoch, R. Hounsell, M. Kato, J.-U. Ness, J. P. Osborne, K. L. Page, V. A. R. M. Ribeiro, P. Rodriguez-Gil, A. W. Shafter, M. M. Shara, I. A. Steele, S. C. Williams, A. Arai, I. Arcavi, E. A. Barsukova, P. Boumis, T. Chen, S. Fabrika, J. Figueira, X. Gao, N. Gehrels, P. Godon, V. P. Goranskij, D. J. Harman, D. H. Hartmann, G. Hosseinzadeh, J. Chuck Horst, K. Itagaki, J. Jose, F. Kabashima, A. Kaur, N. Kawai, J. A. Kennea, S. Kiyota, H. Kucakova, K. M. Lau, H. Maehara, H. Naito, K. Nakajima, K. Nishiyama, T. J. O'Brien, R. Quimby, G. Sala, Y. Sano, E. M. Sion, A. F. Valeev, F. Watanabe, M. Watanabe, B. F. Williams, Z. Xu. M31N 2008-12a - the remarkable recurrent nova in M31: Pan-chromatic observations of the 2015 eruption. The Astrophysical Journal. 833, 149 (2016).
M. Henze, J.-U. Ness, M. J. Darnley, M. F. Bode, S. C. Williams, A. W. Shafter, G. Sala, M. Kato, I. Hachisu, M. Hernanz. A remarkable recurrent nova in M 31: The predicted 2014 outburst in X-rays with Swift. Astronomy & Astrophysics. 580, A46. (2015)
M. Henze, J.-U. Ness, M.J. Darnley, M.F. Bode, S.C. Williams, A.W. Shafter, M. Kato, I. Hachisu. A remarkable recurrent nova in M31 - The X-ray observations. Astronomy & Astrophysics. 563, L8. (2014)